When you play a class for the first time you start thinking about how totally awesome it's going to be and how special you are. You run around for the first 20 levels eager to show everyone else how super cool your new class is.
If you're a warrior you jump in and save that priest from the mob of 3 striders...and she runs away instead of healing you.
If you're a druid you start to heal that protection pally who's pulled 5 to many bristlebacks...then you lose agro, he heals himself and finishes off the 2 that killed you.
Eventually you ding 31 and start to realize that your class isn't all that special...but that shaman that just owned your face is!
...and you re-roll.
By the time you hit end game all you really want is a class that can heal, kill or survive...just like everyone else.